1. Field of the Invention
This invention relates to systems for monitoring the condition of materials. More particularly, this invention relates to condition monitoring systems which are responsive to a plurality of material conditions. Still more particularly, this invention relates to condition monitoring systems which may be used to monitor materials which can leave a coating on condition--responsive apparatus. Even more particularly, this invention relates to condition monitoring systems providing an output relating to the degree to which the condition--responsive apparatus is coated.
2. Description of the Prior Art
Many systems are known which are responsive to the condition of materials, such as the presence or absence of materials at a monitored location. Such systems are often used as level controls, e.g. level switches including high level alarms. One well-known monitoring technique for high-level alarms involves positioning a material responsive sensor including a measuring electrode in a vessel and monitoring the electrical properties of that electrode with respect to a reference electrode, usually the grounded vessel wall. Such monitoring is usually of the conductive and/or susceptive portions of the admittance between the electrodes, and may be performed in a radio frequency bridge or other admittance--responsive circuit. Such monitoring operates on the principle that there is a detectable increase in admittance between the electrodes, with respect to its empty--vessel value, when the material level increases to cover the measuring electrode.
However, it has long been recognized that such admittance--responsive systems are subject to erroneous high-level indications, which result from the fact that the admittance presented to the sensor by a coating of material may be substantially as large as the admittance presented when the sensor is fully covered. To minimize this effect, systems have been devised which have the capacity to ignore substantial coating of the sensor. Such a system is disclosed in U.S. Pat. No. 3,706,980 and involves the use of a "guard" (or "shield") electrode disposed between the measuring electrode and the reference electrode, which guard is driven at substantially the potential of the measuring electrode to reduce surface leakage current along the sensor. However, such systems are still subject to a false high-level indication; when the coating thickness increases sufficiently, the guard is not sufficiently effective to permit that condition to be distinguished from a true high-level condition. This is an intrinsic attribute of admittance--responsive systems.
Another aspect of many known systems is that they are two-state systems: their output is a high or low logic signal, a closed or open relay contact, a lamp which is on or off, or some similar bi-level output, which is intended to correspond with and represent the presence or absence of material in the vicinity of the sensor, e.g. high material level or low material level conditions. A coated sensor, which is in fact a range of third conditions, is by design or by default assigned to one of the two available output states. Typically, a coating of sufficiently great thickness and conductance or susceptance will provide the same output state as a full vessel. The system may be designed to "ignore" up to a certain amount of coating, but there is always some coating condition to which it must falsely respond.
The foregoing aspects of the known systems can represent severe drawbacks in certain applications, particularly where coatings can accumulate, i.e. grow in thickness with the passage of time. In such applications, the system may respond properly to material when installed, and even for a time as a coating begins to accumulate. Eventually, however, at some unknown time the system will give a false high-level output when its coating tolerance has been exceeded. Such an application is in monitoring the lavatory waste holding tanks of airplanes. In some airplanes, the output of the waste tank monitoring system is used to lock the lavatory doors to prevent their further use. False high alarms can needlessly eliminate the ability of the passengers to eliminate, to their great discomfort on long flights. Other applications are subject to inconvenience, expense, or hazard upon coating--induced false level signals.
It would be highly desirable for a condition monitoring system to determine the extent of coating present on members in the interior of the vessel, and in particular, on the sensor, in order that the coating could be cleaned prior to building up sufficiently to interfere with level or other measurements desired to be made in the presence of a coating. However, known bi-level output admittance-response systems are incapable of doing so. This is because the admittance presented to the sensor is a function of the material electrical properties as well as the condition of the material in the vicinity of the sensor. Because the material electrical properties may vary widely and in an unknown fashion between applications, over time in a single application, or even from place to place within a vessel, it is not possible in principle to devise an admittance-responsive system which can unambiguously distinguish between a coated sensor and a true high level, covered sensor condition. As used herein, an admittance-responsive system is one which responds to the admittance presented by the monitored material to the sensor electrode.
Systems for monitoring the condition of materials (e.g. material level, proximity, etc.) at a plurality of discrete condition points are also known. These systems generally fall into two categories.
Systems of the first category generally operate by converting a measurable variable, such as capacitance or admittance, which is related to a material condition being studied, such as height, into an analog signal related to the measured variable. The analog signal is then compared to a plurality of discrete signals or converted to a digital signal which is compared with the plurality of discrete values. In one multi-point monitoring system of the first category known to the inventors, radio frequency admittance measurements are converted to an analog signal which is routed to a plurality of current sensing set point relays. A major problem with such systems is that each stage of operation upon the measured variable, such as amplification, conversion, or comparison, has the potential of introducing spurious effects. In combination, these effects may be sufficient to generate a false output at the comparison stage. It would be necessary to use relatively expensive, very high quality circuit components with very narrow operating tolerances to prevent the introduction of spurious effects.
Systems of the second type incorporate a bridge which compares the measured variable corresponding to the material condition being studied with a reference variable. The output of the bridge is thereafter processed to provide a useful signal. The major advantage of this type of system over the first is that the introduction of spurious effects from the processing circuitry has limited impact because the critical step, bridge comparison, occurs before amplification and subsequent signal processing steps. Known multiple set point monitoring systems which incorporate a bridge are typically operated at balance at a single point and off balance for other points. The balance point may or may not be a set point of interest.
A problem in all known multiple set point systems of both categories is that potentially interfering signals may be present either where the material is being measured or where the resulting measurement signal is processed. It may be necessary to guard the measurement signal against these interfering signals to prevent a false reading of the system. It is considerably more difficult to guard a signal which takes on a wide range of values than it is to guard a signal which is only of interest at a single value. This particular difference is so significant that it allows relatively inexpensive, single-point monitoring systems to outperform more expensive, analog monitoring systems in which interfering signals are present. Such signals might arise, for example, by residual coatings of conductive materials on the sensing element in radio frequency admittance-type material level systems.
Single set point bridge systems are desirably balanced near the set point being examined. In single set point guarded systems, the reference potential of the bridge typically provides a low impedance source of voltage which will be the same as the sensing element voltage when the bridge is balanced. This allows the reference potential to be effectively used to provide a guard voltage to a guard electrode shielding the sensing element. Unfortunately, when the bridge is not balanced, the voltage at the sensing electrode departs from the reference potential. This renders a guard electrode coupled with the reference potential less effective.
A number of other single set point, material condition monitoring systems are known which are capable of generating a plurality of sequential reference signals for calibration. For example, commonly assigned U.S. Pat. No. 4,485,673 to Stern discloses a single set point two-wire level measuring system utilizing a pair of admittance sensitive sensors, each including a capacitive balance bridge. A pair of set point calibration systems is provided, each with a multiplicity of capacitances. The capacitances are selectively coupled to each of the two bridge networks provided for set point calibration. The system is thereafter operated with fixed reference capacitances. U.S. Pat. No. 4,555,941 describes a material level detection system incorporating an automatic calibration circuit in which capacitances are automatically switched into an LC or resonant circuit. Both systems are unsuited for multi-set point operation in their present configuration. Each lacks means for changing the reference set point during monitoring operations. Each also lacks a feedback loop which would adapt to changes in the reference set point during monitoring operations.
U.S. Pat. No. 4,063,447 to Mathison discloses a monitoring system having a bridge network in which a reference current source is adjusted automatically to compensate to system drift. It is also unsuited for multi-set point operation in its given configuration.
U.S. Pat. No. 4,383,444 discloses a capacitance level detection system of the first category in which a measurement capacitance is processed and then compared to a reference capacitance. A microprocessor is provided as part of the system for automatic calibration. There is no teaching or suggestion that this system is useful for or capable of multi-set point operation. Being a first category type system, it is also susceptible to spurious effects during measurement and signal processing.
Lastly, adjustable differential set point monitoring systems are known. Such systems have typically been provided for monitoring sumps and the like. A bridge is provided with a material condition responsive sensor for developing a variable, material condition dependent admittance. A primary capacitor provides a reference admittance. When the bridge output switches as a result of low material related admittance from the sensor, which occurs when the materials in the sump falls below a first level, a second capacitor is coupled with the first capacitor to raise the reference admittance. The bridge output will not switch until the material has risen to a second level above the first. The single output signal of such systems is indefinite as to a particular set point as it may be related to either of two reference capacitances.